readme about strings

[lttv.git] / genevent-new / README

Mathieu Desnoyers -- November 2005

This is a complete rework of genevent.

The 'genevent' program parses event descriptions and generates 
the inline functions to record events in the kernel.

There are several files in the directory:
 genevent.c, genevent.h, crc32.tab, parser.c and parser.h

In fact, crc32.tab, parser.c and parser.h are the same files as  
those in LTT library.

Note about strings :
There are three methods to write strings in genevent, each suitable and
efficient for a particular case. They are explained here from the fastest
to the slowest.
1 - The C code presents a fixed size string.
	For example, you find :
		char mystring[10];
	as string definition.

	you must then define it as an array of char :
	<array size=10/><char></array>

	Note, however, that you might not want to declare a fixed size for trace size
	and unnecessary copy matters.
	
	For instance, on a 32 bits architecture, copying a n bytes array takes
	approximately* n/4 memory read and write, for n/2 memory operations.
	
	Using the	slower method described in (3), with a strlen and memcpy, where
	"u" is the number of used caracters, takes u+1 reads for the strlen, and
	approximately* (u+1)/4 read and write for the memcpy, for a total of :
	(3/2)*(u+1) memory access.

	So, if (n/2) > (3/2)*(u+1), or : n > 3*u+3
		where n is the size of the array
					u is the average number of used caracters (excluding the \0)
				it becomes faster to use the method number 3 with strlen.

2 - The C code presents a variable size string together with its
		size.

	A typical use for this case is filenames in the Linux kernel. The
	dentry strucure has a d_name members, which is a struct qstr containing
	a unsigned int len and const unsigned char *name.

	you must use a sequence to declare this efficiently :
	<sequence lengthtype=<uint>/><char></sequence>
		
3 - The C code presents a \0 terminated string.
	
	This is the slowest, but most convenient way to declare a string. You are
	discouraged to use it when options 1 or 2 are available. It will dynamically
	calculate the string length (byte by byte read) and only afterward do a
	memcpy.

	Note that, as explained in 1, if n > 3*u+3, it becomes faster to use this
	method instead of copying the whole fixed size array.

	Declare like this :
	<string>

Here is a brief description of how to use genevent.

make
make install


* Add new events to the kernel with genevent

su -
cd /usr/local/share/LinuxTraceToolkitViewer/facilities
cp process.xml yourfacility.xml
  * edit yourfacility.xml to fit your needs.
cd /tmp
/usr/local/bin/genevent /usr/local/share/LinuxTraceToolkitViewer/yourfacility.xml
cp ltt-facility-yourfacility.h ltt-facility-id-yourfacility.h \
         /usr/src/linux-2.6.12-rc4-mm2-lttng-0.2/include/linux/ltt
cp ltt-facility-loader-yourfacility.c ltt-facility-loader-yourfacility.h \
         /usr/src/linux-2.6.12-rc4-mm2-lttng-0.2/ltt
  * edit the kernel file you want to instrument
    - Add #include <linux/ltt/ltt-facility-yourfacility.h> at the beginning
      of the file.
    - Add a call to the tracing functions. See their names and parameters in
      /usr/src/linux-2.6.12-rc4-mm2-lttng-0.2/include/linux/ltt/ltt-facility-yourfacility.h



* The approximation comes from the fact that copies of number of caracters non
  multiple of the architecture size takes more operations (maximum of :
	(architecture size (in bytes) - 1) operations).